Over the last few decades, analog integrated circuits have more and more been replaced by digital circuits because digital circuits tend to be less noisy, require a lesser area per complexity. However, the outside world is of analog nature and to be able to interact with computers, etc., some amount of analog circuits, e.g., filters and amplifiers, and mixed analog/digital circuits, e.g., digital-to-analog and analog-to-digital converters, are needed. Moreover, for very high frequencies the design of digital circuits becomes more of an analog nature, since the transistor no longer can be considered as a switch.
The circuit industry has witnessed increasing levels of integration for such applications and CMOS has emerged as the technology most suited to cost-effective, high-volume integration. The CMOS transistor and other important common analog building blocks are used in all high-speed high-resolution circuits. These building blocks are used to design larger more complex circuits, e.g., continuous-time filters, discrete-time filters, power amplifiers, and data converters.
An operational transconductance amplifier (OTA) or its buffered version is a fundamental building block for many switched capacitor and continuous time filters and data converters. Depending on the application, there are different requirements imposed on the OTA. In continuous time filters, for example, where the OTA often operates in an open loop configuration, the linearity and phase response are important. Sampled data circuits usually require a high open loop gain and stability in a closed loop configuration. The demand for fast OTA settling and slewing resulted in sophisticated class AB amplifiers and elegant biasing techniques. OTA dynamic range is becoming an important parameter when migrating to reduced supply voltages. Numerous OTA optimization and compensation techniques exist. However, the OTA that meets the requirements of all applications does not exist. Rather, the OTA remains a handcrafted circuit in most cases.
Differential OTAs have better inherent cancellation of first order CMRR due to their pure differential nature. However, some circuits cannot be implemented using differential OTAs. Implementing a differential circuit using single-ended OTAs places a high demand on CMRR of single-ended OTAs. For example, an ADC may be implemented using single ended large input/output dynamic range OTAs with a high common-mode rejection ratio (CMRR) and open loop gain. Single-stage OTAs are power efficient for high-speed applications. Three basic configurations of the single-stage OTAs are available, namely the telescopic, current mirror and folded cascode topologies. Contrary to the fully differential OTA, the single ended current mirror OTA does not have first order cancellation of CMRR. Thus, while a high open loop gain, e.g., 66 dB, can be achieved using standard techniques, target high CMRR is a design challenge.
It can be seen then that there is a need for a method and apparatus for providing high CMRR in a single-ended CMOS operational transconductance amplifier.